This invention relates to a .circle. for effecting a fluid seal between a couple of members one of which is movable with respect to the other, comprising an outer annular stressing section radially biasing against a concentric inner sealing section, along an outer surface thereof.
More particularly, the present invention refers to a mono-directional composite seal (MCS) for reciprocating motions which finds a preferred use as a packing for hydraulic cylinders for effecting a fluid seal around piston rods.
The invention is applicable to sealing ring structures comprising a tough elastic sealing ring which may be of any suitable thermoplastic substance, i.e. a polymer selected from the group comprising: polytetrafluoroethylene, ultrahigh-molecular-weight polyethylene (UHMW-PE), polyester elastomers and a separate highly elastic stressing ring for pressing radially on the sealing ring and preferable consisting of an elastomeric material such as rubber, or to integral structures having a sealing section produced from an elastic material or a combination of materials to provide a tough elastic sealing section and an integral highly elastic stressing section.
In the case of piston rod seals of the above nature and, more generally, in the case of seals interposed between a stationary and a movable member, it is necessary to control and maintain to a limited amount the drag flow of the hydraulic fluid during reciprocating displacement.
To this end, the sealing ring structures are provided with a sealing ring and a stressing ring, or a sealing section and a stressing section in the case of integral structures, which are commonly situated together within a groove or recess of the housing (for a rod seal) or of the piston (for a piston seal).
Typical examples of sealing ring structures of the above kind are described in U.S. Pat. Nos. 3,942,806 and U.S. Pat. No. 4,449,718.
These known structures comprise a stressing ring consisting of a rubber-elastic ring of circular (O-ring) or rectangular cross-section, which extends coaxially with and surrounds the sealing ring which generally has a substantially rectangular cross-section.
As a result of radial prestressing of the stressing ring between the bottom of the groove and the external surface of the sealing ring in contact with the stressing ring, the following effects are achieved:
a) a dynamic sealing between the stationary and movable members, for example between the stationary part of a cylinder and a reciprocating rod;
b) the secondary leakage path between the sealing ring and the bottom of the groove and the sealing ring is closed.
As a general rule, the dynamic sealing between the stationary and reciprocating members is obtained by shaping the inner surface of the sealing ring so as to have an annular sharp edge which provides, in operation, a contact surface as limited as possible with the movable member.
This annular edge, adapted to slidably engage the movable member, is defined by the intersection of a recess open towards the high pressure side of the seal and a radially inclined conical surface open towards the low pressure side of the seal.
This contact surface, generates a very steep rise of the contact pressure, i.e. a high pressure gradient, between the sealing ring and the mating surface of the movable member, with respect to the direction from the high pressure side to the low pressure side of the seal. The high pressure gradient thereby generated limits the drag flow of the hydraulic fluid which takes place when the movable member moves outward (i.e. towards the low-pressure side of the seal) with respect to the stationary member.
The radially inclined conical surface makes on the contrary low the pressure gradient between the sealing ring and the mating surface of the movable member, with respect to the direction from the low pressure side to the high pressure side of the seal. Consequently, this arrangement favours the dragging of the hydraulic fluid layer back again into the stationary member when the movable member moves inwards.
Practical experience has shown that conventional MCS performance is affected by a series of drawbacks.
A first of such drawbacks is that the sealing ring is forced to rotate toward the low-pressure side of the seal by the action of the force exerted onto the outer surface thereof by the stressing ring, as well as by the hydraulic fluid pressure applied to its high-pressure surface.
As a consequence, the length of the contact surface which engages the movable member, may be extended up to a value which allows for a noticeable leakage to take place.
A second drawback of conventional MCS is the progressive plastic deformation of the sealing ring cross section caused by the pressure applied on its outer surfaces. This deformation is particularly noticeable in proximity of an outer edge of the radially inclined conical surface facing the low pressure side of the seal.
This deformation may in some cases result in the extrusion of the sealing ring material into the clearance volume between the stationary member and the moving member. This phenomenon could give rise in turn to breakages of the extruded material in fragments which affect the seal integrity and the seal performance.